Method of manufacturing rare-earth bond magnet

ABSTRACT

A method of manufacturing a rare-earth bond magnet is provided, by which magnetic properties and mechanical properties of a magnet are improved. In a method of manufacturing a rare-earth bond magnet, a mixture including rare-earth magnet powder, thermosetting resin, and an additive is subjected to compression molding, and a compact is irradiated with a microwave, so that the thermosetting resin is cured using heat generated by the rare-earth magnet powder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a rare-earth bond magnet.

2. Description of the Related Art

A rare-earth magnet has been used for a material for a resin-bonding bond magnet. The resin-bonding bond magnet is manufactured by compression molding of rare-earth magnet powder being mixed with a resin binder, and has advantages that the bond magnet is high in dimension accuracy compared with a sintered magnet, and can be molded even into a complicated shape, and has a high yield.

As the rare-earth magnet to be a material of the rare-earth bond magnet, for example, a Sm—Co magnet including Sm₂Co₁₇, an Nd—Fe—B magnet, and a Sm—Fe—N magnet are given. The Sm—Co magnet, which is known as an expensive material, is high in heat resistance, and therefore used in the case that high heat resistance is required.

The Nd—Fe—B magnet is bad in heat resistance and corrosion resistance, but the magnet is known as a highly magnetic material although it has isotropic magnetization, in which magnetic directions are randomly oriented, given by a liquid quenching method. Furthermore, a magnet is known, which is made into anisotropic magnet powder by a so-called HDDR method (Hydrogenation Decomposition Desorption Recombination Method) so as to have higher magnetic properties. Recently, the Sm-Fe-N magnet is also focused with attention as a rare-earth magnet material that has a high magnetic properties equivalent to the above magnetic properties, and is comparatively inexpensive.

A bond magnet using such a rare-earth magnet as a material is used in many fields such as audio/video devices, rotary devices, communication devices, measurement devices, and car components. With increase in demand, the bond magnet is increasingly required to be improved in magnetic properties, in addition, required to be improved in industrial productivity, mechanical strength, corrosion resistance and the like.

However, when the amount of the rare-earth magnet material is increased to improve the magnetic properties, the amount of thermosetting resin for binding the rare-earth magnet material is decreased, resulting in a phenomenon of reduction in mechanical strength. Conversely, when the amount of the thermosetting resin is increased to increase strength, the magnetic properties are degraded. Therefore, at present, the amount of thermosetting resin is increased to increase strength of a magnet even if reduction in magnetic properties is induced.

Moreover, there is a problem that when the bond magnet is heated by a special heat treatment furnace to cure a resin binder as in the past, one to two hours is taken for completing self-combustion of the bond magnet, consequently long time is required for curing. Furthermore, there is a problem that while polymerization of thermosetting resin proceeds in a region near an outer surface of a rare-earth bond magnet, the thermosetting resin cannot be adequately polymerized in the inside of the bond magnet, leading to reduction in strength.

Moreover, it is necessary that oxidation of a rare-earth magnet is suppressed during curing to prevent reduction in magnetic properties. However, when the special heat treatment furnace is used, oxygen is hard to be perfectly removed in the light of a configuration of the heat treatment furnace. To cope with this, oxygen was substituted by an inert gas such as high-purity nitrogen or argon gas, or evacuation to a vacuum was performed. However, in such a case, much time and cost were taken, consequently production efficiency was reduced, in addition, target magnetic properties were hardly obtained.

To cope with this, a method is proposed, in which a surface of magnet powder is uniformly covered with resin, thereby oxidation of the magnet powder is suppressed during a manufacturing process, so that improvement in magnetic properties and mechanical strength is achieved (refer to JP-A-6-302418).

Moreover, a method is proposed, in which curing of a binder is performed by near infrared heating, thereby processing time is reduced so that oxidation of a magnet is suppressed, and consequently improvement in magnetic properties is achieved (refer to JP-A-9-180920).

However, in the method of covering the magnet powder with resin, since the special heat treatment furnace is used, and a specimen is heated by radiation or heat conduction from an external heat source, processing time becomes long. Moreover, temperature of the specimen is high in a region near the heat source, and low in a region away from the heat source, which induces a phenomenon that an uncured portion exists in thermosetting resin, leading to inadequate strength of a bond magnet. Furthermore, since it is extremely difficult to perfectly uniformly cover the surface of the magnet powder, reduction in magnetic properties may be caused due to oxygen remained in the heat treatment furnace.

On the other hand, in the method of performing the near infrared heating, since near infrared rays may not be adequately transmitted to the inside of a magnet compact, there is a possibility of occurrence of a phenomenon that an uncured portion is formed in thermosetting resin particularly in the inside of a specimen.

The invention was made in the light of the above problem, and an object of the invention is to provide a method of manufacturing a rare-earth bond magnet, by which magnetic properties and mechanical properties of a magnet are improved.

SUMMARY OF THE INVENTION

To solve the problem, a first aspect of the invention is summarized in that a mixture including rare-earth magnet powder and thermosetting resin is subjected to compression molding, and a compact given by the compression molding is irradiated with a microwave, so that the thermosetting resin is cured using heat generated by the rare-earth magnet powder.

A second aspect of the invention is summarized in that in the method of manufacturing a rare-earth bond magnet of the first aspect, the compact in which the thermosetting resin is cured by irradiating the microwave is cooled in a vacuum or in an inert gas.

A third aspect of the invention is summarized in that in the method of manufacturing a rare-earth bond magnet of the first aspect, the microwave irradiated to the compact has a frequency of 1 GHz to 30 GHz.

A fourth aspect of the invention is summarized in that in the method of manufacturing a rare-earth bond magnet of the second aspect, the microwave irradiated to the compact has a frequency of 1 GHz to 30 GHz.

A fifth aspect of the invention is summarized in that in the method of manufacturing a rare-earth bond magnet of any one of the first to fourth aspects, the compact is irradiated with a microwave at an atmosphere containing nitrogen atoms, so that nitriding of the rare-earth magnet powder and curing of the thermosetting resin are concurrently carried-out.

A sixth aspect of the invention is summarized in that in the method of manufacturing a rare-earth bond magnet of any one of the first to fourth aspects, average grain diameter of the rare-earth magnet powder is 2 to 150 μm.

According to the first aspect of the invention, the mixture including the rare-earth magnet powder and the thermosetting resin is subjected to compression molding, and a compact is irradiated with a microwave. Thus, the rare-earth magnet powder is self-heated, -and thus heat is transferred to thermosetting resin, which is adhered in a gap between rare-earth magnet particles or on a surface of the particles, consequently a specimen as a whole can be uniformly heated. Therefore, the thermosetting resin can be instantaneously cured, and processing time can be reduced.

According to the second aspect of the invention, since after the microwave irradiation is performed, the compact is cooled in a vacuum or in an inert gas, oxidation of the rare-earth magnet powder can be suppressed, and consequently excellent magnetic properties can be kept.

According to the third or fourth aspect of the invention, since the microwave irradiated to the compact has a frequency of 1 GHz to 30 GHz, occurrence of arc discharge can be suppressed, and the compact can be heated to a temperature within a desired temperature range.

According to the fifth aspect of the invention, since nitriding of the rare-earth magnet powder and curing of the thermosetting resin are concurrently carried out, processing time can be reduced.

According to the sixth aspect of the invention, since powder having average grain diameter of 2 to 150 μm is used as the rare-earth magnet powder, oxidation of magnet powder can be suppressed, and when the magnet powder is molded while aligning magnetization directions, particles can be aligned in a desired magnetization direction.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a diagram showing an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a diagram showing an embodiment of the invention.

A microwave oscillating tube 1 that oscillates a microwave is connected to an applicator 3 via a wave guide 2. The microwave oscillated by the microwave oscillating tube 1 is transmitted to the applicator 3 through the waveguide 2. The waveguide 2 has an isolator 4. The isolator 4 transmits a microwave on the waveguide 2 only to a direction of the applicator 3, and absorbs a microwave transmitted in an opposite direction.

A specimen 5 is placed in the applicator 3 and irradiated with the microwave. The applicator 3 is a closed vessel made of metal, and is formed so as to prevent leakage of the microwave to the outside. Moreover, the applicator 3 is connected with a gas supply source 6 for introducing an inert gas such as nitrogen. Furthermore, the applicator 3 is connected with a pump 7 for exhausting atmosphere from the inside thereof.

The specimen 5 is connected to a thermocouple 8, so that temperature change of the specimen 5 associated with microwave irradiation can be measured. The applicator 3 has a pressure gauge 9, so that internal pressure can be measured. The microwave oscillating tube 1, gas supply source 6, pump 7., thermocouple 8, and pressure gauge 9 are connected to a controller 10, and thus controlled respectively. Thus, atmosphere and pressure within the applicator, increase in temperature of the specimen, and the like can be controlled by the controller 10.

As the microwave oscillating tube 1, magnetron, gyrotron, klystron and the like can be used.

A method of manufacturing a rare-earth-element/transition metal-based (hereinafter, called R-TM-based), rare-earth bond magnet is described for each step below. R shows at least one or two elements among rare earth elements, and TM shows at least one or two elements among transition elements.

(1) Rare-Earth Magnet Powder

As the rare earth element configuring the R-TM-based alloy of the invention, Y (yttrium) and lanthanide elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) can be preferably used. Particularly, when Pr, Nd or Sm is used, magnetic properties can be remarkably improved. Moreover, at least two kinds of rare earth elements are used in combination with each other, thereby residual magnetic flux density and coercive force of the magnetic properties can be improved.

Specifically, Sm-Co magnet powder including SmCo₅ or Sm₂Co₁₇, or Nd—Fe—B magnet powder including Nd₂Fe₁₄B manufactured by the HDDR method or the liquid quenching method can be used. Alternatively, Sm—Fe—N magnet powder can be used, which contains, as basic components, rare-earth elements mainly including Sm, transition metals mainly including Fe, and interstitial elements mainly including N. Alternatively, at least two kinds of the above rare-earth magnet powder may be mixedly used.

In manufacturing of the R-TM-based or R-TM-N-based rare-earth magnet powder, in the case of a typical melt-casting method, the rare-earth metal, transition metal, and the like are mixed in a predetermined mixing ratio and then subjected to high-frequency melting in an inert-gas atmosphere. Furthermore, in the manufacturing, an obtained alloy ingot is subjected to heat treatment, and then crushed into a predetermined grain size by a crusher such as a jaw crusher, a jet mill, or an attritor.

In the liquid quenching method, in contrast to the alloy ingot produced as above, molten alloy is discharged onto a roll being rotated at high speed so as to be contacted to an outer circumferential surface of the roll, thereby the molten alloy is quenched to produce an alloy ribbon. The alloy ribbon is crushed into a predetermined grain size by the crusher. Even if C, Bor the like is mixed in the alloy as an inevitable impurity during such melting, no particular problem occurs.

The magnet powder preferably has a grain diameter of 2 to 150 μm in average. When the average diameter is less than 2 μm, the powder is easily oxidized, in addition, increase in density due to agglomeration or spring back is not achieved during producing a bond magnet, resulting in degradation in magnetic properties. Moreover, in the case that the average diameter is more than 150 μm, when the magnet powder is molded while a magnetic field is applied to align magnetization directions, particles are not oriented in a desired magnetization direction, which induces degradation in magnetic properties.

(2) Thermosetting Resin

As thermosetting resin, while it is not particularly limited, phenolresin, polyesterresin, epoxyresin, urearesin, melamine resin and the like can be used. In the respective kinds of thermosetting resin, in either of one-component type and two-component type, or in either of liquid and solid, any kinds of resin can be used in combination with one another, as long as the resin shows a curing reaction upon heating. The adding amount of the resin is preferably in a range of 0.5 wt % to 3.0 wt % to the amount of magnet powder. When the amount is less than 0.5 wt %, a compact is insufficient in strength, and when it exceeds 3.0 wt %, a volume ratio of the rare-earth magnet powder is decreased, causing degradation in magnetic properties.

(3) Additive

Regarding an additive, while it is not particularly limited, a surfactant, a coupling agent, a lubricant, a mold lubricant, a frame retarder, a stabilizer, inorganic filler, pigment and the like can be used for the additive. As the additive, any agent can be used, if it provides flowability required for filling a mixture into a die, slidability required for aligning magnetization directions by applying a magnetic field, or mold-releasability required in the case of removing a compact from a die, or if it gives water repellency, improvement in density, or improvement in strength to a compact. Moreover, a plurality of kinds of additives may be used in combination with one another.

(4) Mixing

The rare-earth magnet powder and thermosetting resin and/or additive are dispersedly mixed by the attritor, a Henschel mixer, or a V blender so as to be pelletized, thereby a compound is obtained. Particularly, an organic solvent or the like is preferably used for mixing and deaeration to uniformly mix the thermosetting resin and the additive before preparing pelletized particles.

(5) Compression Molding

A pressing machine is used, in which an electromagnet is provided in a die for applying a magnetic field, and the compound is filled in the die, then the compound is subjected to compression molding at a pressure of 10 ton or more in a magnetic field of 10 kOe (oersted) or in no magnetic field.

While an isotropic magnet material prepared by the liquid quenching method may be molded in no magnetic field, in the case of rare-earth magnet powder including an anisotropic magnetic material, since when a magnetic field is less than 10 kOe, the magnet powder is not oriented in a magnetization direction, a magnetic field of 10 kOe or more is necessary for the rare-earth magnet powder. Moreover, when the rare-earth magnet powder is molded at increased die temperature of 50 to 150° C., thermosetting resin is melted, which leads to increase in density, consequently molding pressure can be reduced, and durability of a die can be improved. When the die temperature is less than 50° C., the thermosetting resin is not melted, and when it is more than 150° C., a compound is agglomerated and solidified before it is put into the die.

(6) Curing of Thermosetting Resin

In the embodiment, when the compression molding is finished, an obtained compact is irradiated with a microwave, so that the thermosetting resin is cured. The rare-earth magnet powder is selectively and rapidly self-heated in this way, thereby the powder is heated to 150 to 300° C. in several minutes. Heat generated by self-heating of the rare-earth magnet powder is transferred to thermosetting resin, which is adhered in a gap between magnet particles or on surfaces of the particles, consequently the thermosetting resin is instantaneously cured. In this process, a specimen as a whole is uniformly heated, thereby a rare-earth bond magnet can be obtained, which has no uncured portion and thus has high strength. Moreover, since the specimen can be heated to a predetermined temperature in several minutes by microwave irradiation, processing time can be reduced, and oxidation of alloy powder can be suppressed.

The microwave irradiated to the compact preferably has a frequency of 1 GHz to 30 GHz. When it is less than 1 GHz, arc discharge tends to occur, and when it is more than 3.0 GHz, it is heated to a desired temperature or more. While atmosphere may be any of air, a vacuum, and an inert gas such as nitrogen gas, the vacuum or inert gas is preferable in the light of suppressing oxidation of a magnet.

Furthermore, in the case that curing of the thermosetting resin and nitriding of the rare-earth magnet powder are concurrently performed, atmosphere having a nitrogen pressure of 0.1 to 5 MPa is preferable. When the pressure is less than 0.1 MPa, nitriding does not penetrate into the inside of a particle and is remained only in a surface. When the pressure is more than 5 MPa, excessive nitriding occurs in a surface of a particle.

Moreover, a compact is heated to a temperature in a range of 250 to 400° C. by irradiation of a microwave, thereby the rare-earth magnet powder can be optimally nitrided. When heating temperature is less than 250° C., nitriding does not proceed, which is not preferable. When it is more than 400° C., the thermosetting resin is decomposed, which is not preferable.

Moreover, as the rare-earth magnet powder to be nitrided, magnet powder including R-TM-based alloy as a major component is preferably used, and for example, rare-earth magnet powder including Sm—Fe or Nd—Fe can be used.

(7) Cooling

After microwave irradiation is performed, a compact in which the thermosetting resin is cured is subjected to cooling. That is, when irradiation of a microwave is finished, while the rare-earth magnet powder itself is rapidly cooled, some oxidation cannot be prevented. To cope with this, a method was attempted, in which the powder was cooled while irradiation output power of the microwave was gradually decreased. However, when the output power is decreased to a certain level or lower, an oxidation reaction becomes preferential, and reduction in magnetic properties is found although the reduction is slight. Therefore, the powder needs to be cooled to room temperature in a vacuum or in an inert gas such as nitrogen gas and argon gas, and external cooling is preferably performed together in some case.

According to the embodiment, the following effects can be obtained.

(1) In the embodiment, a mixture including the rare-earth magnet powder, thermosetting resin, and additive is subjected to compression molding, and a compact is irradiated with a microwave. Thus, since the rare-earth magnet powder can be selectively self-heated, and surrounding thermosetting resin can be cured using heat generated by the magnet powder, a specimen as a whole can be uniformly heated so that an uncured portion is eliminated, thereby mechanical strength can be improved. Moreover, since the uncured portion is eliminated and thus mechanical strength can be improved, the amount of rare-earth magnet material can be increased in a bond magnet, leading to improvement in magnetic properties. Furthermore, since the thermosetting resin can be instantaneously cured using heat generated by the rare-earth magnet powder, processing time can be reduced. In addition, oxidation caused by long heating is suppressed, thereby reduction in magnetic properties can be prevented.

(2) In the embodiment, since a compact in which thermosetting resin is cured is cooled in a vacuum or in an inert gas, oxidation of the rare-earth magnet powder can be suppressed, consequently excellent magnetic properties can be kept.

(3) In the embodiment, frequency of a microwave irradiated to a compact is made to be in a range of 1 GHz to 30 GHz. Therefore, arc discharge that tends to occur at low frequency can be suppressed. Moreover, the compact is prevented from being heated to a desired temperature or more due to excessively high frequency, consequently the compact can be heated to a temperature in a desired temperature range.

(4) In the embodiment, a microwave is irradiated to a compact at a nitrogen atmosphere and at a pressure of 0.1 to 5 MPa, thereby nitriding of rare-earth magnet powder and curing of thermosetting resin can be concurrently carried out. Therefore, processing time can be reduced compared with a case that a nitriding step and resin curing are separately carried out.

(5) In the embodiment, average grain diameter of rare-earth magnet powder is made to be 2 to 150 μm. Therefore, oxidation of a magnet due to increase in surface area of the magnet can be suppressed, and when the magnet powder is molded while aligning magnetization directions, particles can be aligned in a desired magnetization direction. 

1. A method of manufacturing a rare-earth bond magnet: compression molding a mixture of rare-earth magnet powder and thermosetting resin, and irradiating a microwave whereby the resin is cured by heat generated by the powder.
 2. The method of manufacturing a rare-earth bond magnet according to claim 1: further comprising cooling by vacuum or an inert gas.
 3. The method of manufacturing a rare-earth bond magnet according to claim 1: wherein the frequency of the microwave is 1 GHz to 30 GHz.
 4. The method of manufacturing a rare-earth bond magnet according to claim 1: wherein the average grain diameter of the rare-earth magnet powder is 2 to 150 μm.
 5. The method of manufacturing a rare-earth bond magnet according to claim 1: wherein the irradiating is irradiated at an atmosphere containing nitrogen atoms. 